Hi Peter:
> After an Abnormal Test for UL1950/EN60950, should the Electric Strength be
> done a) immediately after the Abnormal Test or b) after Abnormal Tesrt when
> unit has cooled down to room temperature c) after Abnormal Test when unit
> has cooled down to Normal Heating Temperature Levels.
As I recall, the electric strength is performed immediately
after the abnormal test.
However, the time from the end of the abnormal test to the
initiation of the electric strength test should not be
critical to the results.
If time is critical to the results, then the implication is
that the electric strength of the insulation is somehow a
function of temperature.
I would not expect this to be the case for conventional
organic insulating materials. Organic insulating materials
decompose with heat. The long-chain carbon molecules are
broken into shorter chain molecules and the carbon begins
to predominate. They do not reconstitute as they cool.
Hence, the change of color towards brown and black. Carbon,
of course, is a conductor, and will cause a failure of the
electric strength test. The failure voltage may be higher
when the insulator is cold, but it should nevertheless fail
the electric strength test.
I believe there are some inorganic insulators used in
sheathed heating elements whose electric strength may be
a normal function of temperature. If this is the case,
then damage to the insulator can only be assessed at room
temperature.
Best regards,
Rich
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